Risers are vital to drilling and extracting oil and other materials from underneath the earth's surface. A riser is basically a tube that connects a well head to a control station where extraction, and, frequently, drilling operations are controlled. When extracting oil, natural gas and/or other materials from underneath the bottom of a body of water, such as a sea, ocean or lake, a riser connects the wellhead at the bottom of the sea, ocean or lake to a platform suspended at the surface of the sea, ocean or lake. In such systems, the riser protects the drill string that extends from the platform and through the wellhead, by encasing the drill string between the platform and wellhead. The riser also provides a conduit for drilling-mud to flow from the platform to the wellhead, and thus into the well. Drilling-mud helps control the pressure inside the well that would otherwise substantially drop because of the hole drilled into the earth. The riser also provides a conduit for the oil, natural gas, and/or other materials to flow from the wellhead to the platform where the oil, natural gas and/or other materials can be secured for subsequent use.
Most risers include a main line and one or more auxiliary lines. The main line encases the drill string as it extends from the platform to the wellhead, and contains the drilling-mud and/or oil and/or other materials as they flow to and from the wellhead and the control station. The one or more auxiliary lines are typically located adjacent and outside of the main tube, and encase control lines that extend from the platform to the wellhead. The control lines may be hydraulic lines, electrical and/or pneumatic lines that connect systems at the wellhead, such as a blowout preventer (BOP) that can cap the well in an emergency.
Most risers are assembled in the field by coupling riser sections together. For example, FIG. 1 shows a portion 20 of a riser that includes five riser sections 22 coupled end-to-end. Each section 22 includes a main line 24 and an auxiliary line 26. Each main line 24 includes a main tube 28 and a flange 30 fastened to each of the main tube's ends. Each of the flanges 30 are designed to be mounted to a flange of another main line 24 by bolts to form a section 22 of the riser. The auxiliary line 26 is typically secured to the flanges 30, but may also pass through a hole in two or more of the flanges 30 as it extends adjacent to two main tubes 28. In some risers the auxiliary line 26, similar to the main line 24, may include flanges attached to the auxiliary line's tube and that can be mounted to another flange of another auxiliary line to form a section of a riser.
Because the distance between the control center and the wellhead is often long—especially when the wellhead is located at the bottom of a sea, ocean or lake—most risers include hundreds of riser sections coupled end to end. When the riser is vertically oriented (as shown in FIG. 1), as is common when extending between a platform suspended at the surface of a sea, ocean or lake and a wellhead, the forces that a riser section experiences (and must withstand) include the weight of the other sections above the section, the weight of the water above the section, and the movement of the water through which the riser extends. To withstand these loads many riser sections include steel flanges welded to a steel main tube, and steel auxiliary lines. The steel riser sections with steel auxiliary lines provide much strength, but unfortunately they are also heavy, which causes the riser sections located near the wellhead to bear a significant amount of weight. To reduce weight, some riser sections include aluminum flanges welded to an aluminum main tube, and aluminum auxiliary lines. The aluminum riser sections, however, provide less strength than the steel riser sections. In response to this, some riser sections include steel flanges welded to a steel main tube, and aluminum auxiliary lines. The combination of a steel main line with an aluminum auxiliary line provides a compromise between high strength and light weight; however, when deeper water depths are experienced, a riser section that provides both, high strength and light weight is needed.